The light output of a semiconductor diode laser is nonlinear with respect to applied drive current. In particular, diode lasers emit a limited amount of light by spontaneous emission when driven by currents below a threshold current, and emit considerably more light by stimulated emission when driven by currents above the threshold current. Moreover, the dependence of light output on drive current is greater for drive currents above the threshold current than for drive currents below the threshold current.
The voltage characteristic of a semiconductor diode laser is also nonlinear with respect to applied drive current. In particular, the voltage across the diode laser saturates for currents above the threshold current.
When used as light sources for optical communications, diode lasers are driven by a signal current combined with a D.C. bias current. The D.C. bias current is selected to optimize the operation of the diode laser for the particular type of signal to be transmitted. For transmission of analogue signals, the D.C. bias current should exceed the threshold current so that the diode laser operates in the most linear region of its light output characteristic, avoiding signal distortion due to the nonlinearity in the light output characteristic around the threshold current. For transmission of digital signals, the D.C. bias current should be near the threshold current so that the nonlinearity of the light output characteristic improves the extinction ratio of the transmitted signal. However, the D.C. bias current cannot be made too much smaller than the threshold current without increasing the time required to turn the diode laser on, and thereby limiting its operation for transmission of high bit rate signals.
Light output characteristics for diode lasers vary with operating temperature and as the diode lasers age. Consequently, the D.C. bias current that should be applied to a diode laser for a particular mode of operation varies with the operating temperature and age of the diode laser. Adaptive control circuits have been developed to sense changes in the operating characteristics of diode lasers and, in response to the sensed changes, to control the D.C. bias current.
Many known adaptive control circuits for controlling the D.C. bias currents of diode lasers sense light output of the diode lasers with back facet monitors, derive voltages which are proportional to the average optical output power of the diode lasers, and adjust the D.C. bias currents to minimize the difference between those voltages and appropriate reference voltages. (See for example U.S. Pat. No. 4,698,817 issued in the name of G. S. Burley on Oct. 6, 1987 and entitled Peak Optical Power Control Circuit for Laser Driver.) Some such diode laser control circuits apply a pilot tone to the bias current and use a resulting pilot tone component of the sensed light output of the diode laser to control a magnitude of a signal current used to modulate the laser. (See for example U.S. Pat. No. 4,504,976 issued in the name of D. Beaudet on Mar. 12, 1985; U.S. Pat. No. 4,995.045 issued in the names of G. S. Burley et al on Feb. 19, 1991; and Japanese Patent Document 60-91687 published in the name of Y. Ogura on May 23, 1985.)
U.S. Pat. No. 4,385,387 discloses a diode laser control circuit which applies a pilot tone to the bias current, senses the light output of the diode laser, and passes the resulting signal through a high pass filter to eliminate low frequency components of the resulting signal. Clipping of the diode laser light output at the threshold current appears in the filtered signal as a reduction in the signal amplitude. The D.C. bias current is adjusted to produce a desired duration of clipping of the diode laser light output which corresponds to a desired relationship between the D.C. bias current and the threshold current. (U.S. Pat. No. 4,385,387 issued in the name of H. Trimmel on May 24, 1983 and is entitled Preconduction Current Control of Diode Lasers.)
In a typical optical fiber communications system, an optical signal is coupled from a diode laser to an optical fiber, and the optical signal is coupled from the optical fiber to a high impedance optical receiver at a remote location. Unfortunately, the pilot tone which is applied to the bias current in some known diode laser control circuits is injected into the fiber as part of the optical signal and can saturate the high impedance optical receiver, adversely affecting the operation of the optical communications system.
U.S. Pat. No. 4,081,670 discloses an adaptive control circuit for biasing a diode laser above threshold for linear operation suitable for transmitting analogue signals. The disclosed control circuit senses an A.C. component of a signal voltage across the diode laser and adjusts the D.C. bias current to minimize the A.C. component of the sensed voltage. (U.S. Pat. No. 4,081,670 issued Mar. 28, 1978 in the name of A. Albanese and is entitled Automatic Bias Control Circuit for Injection Lasers.)